In a process of continuously casting semi-finished products such as steel slabs used for manufacture of the broaden steel plates, a refractory material made immersion nozzle is commonly used for a molten steel path between a tundish containing molten steel and a continuous casting mold. The immersion nozzle is disadvantageous in that, since alumina is liable to be deposited on the inner surface of the nozzle, particularly, in continuous casting for aluminum-killed steels, the molten steel path is narrowed with casting time, which makes it impossible to obtain the desired flow rate of the molten steel.
In general, to prevent the deposition of alumina, an inert gas such an Ar gas is supplied within the nozzle during supplying the molten steel. However, when the discharge speed of the molten steel is larger in high speed casting with high throughput, the inert gas is trapped in the flow of the molten steel and is obstructed from being floated on the molten pool surface within the mold, to be thus trapped in the solidified shell. Because of the inert gas trapped in the steel, there often occur defects such as sliver, blistering and the like in the final products.
Also, in an immersion nozzle of a two-hole type, which includes the right and left symmetric discharge ports at the lower end portion thereof, the flow of the molten steel in the mold is liable to be made uneven by the asymmetric blocking caused in the right and left discharge ports, thereby bringing about the lowering of the quality of the product. In this case, differently from the gas trap, there occur the entrapments of inclusions and mold powder due to a deflected flow generated by the blocking of the discharge ports of the nozzle.
The present inventors have examined the nozzle blocking in continuous casting using a low carbon aluminum-killed steel being mainly deoxidized by Al and containing a carbon concentration of 500 ppm or less. As a result, it was found that the nozzle blocking was almost eliminated by adjusting the oxygen concentration in molten steel to be 30 ppm or less, preferably, 20 ppm or less, and using a pipe-like straight immersion nozzle with the leading edge being opened and served as the discharge port for molten steel. However, such a straight nozzle is disadvantageous in that, since the discharge flow of the molten steel is directed downwardly of the mold, the inclusions and gas babbles in molten steel permeate to the deep portion of the molten steel pool.
To prevent the permeation of the inclusions and the like, there has been proposed such a technique that a static magnetic field generator for applying a static magnetic field to the molten steel is disposed around the continuous casting mold for restricting the flow of the molten steel being directed downwardly. For example, Japanese Patent Laid-open sho 58-55157 discloses a technique of generating a direct current magnetic field in the level near the meniscus around a continuous casting mold, and of adjusting the intensity and direction thereof, thereby controlling the permeation depth and the permeation direction of the pouring flow of the molten steel. However, in this technique, the magnetic field is applied only to the level near the meniscus, and accordingly, the restricting force is insufficient.
The present inventors have established a technique of casting steel slabs excellent in qualities, which comprises the step of adjusting the oxygen concentration in molten steel at a lower value, and using a straight nozzle without injection of Ar gas within the nozzle, thereby preventing the nozzle blocking, while controlling the descending flow of the molten steel by the strong restricting force.
Further, the present inventors have found the following fact: namely, for the meniscus variation which is attributed to the flow of the molten steel toward the meniscus generated by the effect of restricting the descending flow of the molten steel, it is effectively restricted by applying the static magnetic field to the meniscus portion.
A primary object of the present invention is to provide a process of continuously casting steel slabs capable of obtaining the steel slabs excellent in the surface and the internal qualities.
Another object of the present invention is to eliminate the nozzle blocking in continuous casting without using Ar gas.
A further object of the present invention is to provide a technique of continuously casting the steel slabs, which comprises the steps of applying a suitable restricting force to the descending flow of the molten steel, and preventing the meniscus variation caused by the above application.